Customers drive this policy. Make no mistake about that. If it was worth the money dealerships would work on old bikes, but today it's not worth it. Like others have stated, the old rat's nest shows up and they want a show bike returned to them for minimal dollars.
The dealers drive this policy, for Japanese and Italian motorcycles, the service department is a must have nuisance. They are notorious for bad service.
A Harley Service Tech leader makes up to $130,000 in salary per year, while a good mechanic makes close to $40 per hour. Their service departments are properly trained and geared to work on their product despite age and the dealer makes good money from that.
Any reputable motorcycle brand has a flat rate manual going back years and years. A professional service department uses that to give the customer an estimate for the task at hand and they always let the customer know that after inspection they will contact the customer for approval if extra work and costs are needed.
It is the customers decision whether to approve the costs and work or not. For most 10 year old Japanese or Italian motorcycles, if engine rebuild work is needed it could cost more than the bike blue book value. A good dealer would tell that to the customer and guide him towards making the right choice.
Anyone who wants to buy a Japanese or Italian motorcycle and who will rely 100% on the dealer for service in the long term beyond 10 years, should rather go visit the BMW or Harley dealer.